The objective of this project is to understand how cells choose between alterantive developmental pathways in respose to cues intrinsic to the young embryo. The Sxl--da gene system in Drosophila melanogaster controls sex determination and X chromosome dosage compensation. We want to understand how the Sxl gene is regulated in response to the ratio of X chromosomes to autosomes through the mediation of a maternally-synthesized product of the da gene. Work planned for this year includes fine structure analysis of Sxl, detailed functional characterization of mutant Sxl alleles, isolation of a male-viable deletion of the Sxl region, dose effect studies on da and Sxl, determination of the relationship of da and Sxl function to that of other sex-specific second chromosome genes, initiation of biochemical studies on Sxl and on sex-specific protein synthesis during early development, and studies of the transposition of the white-crimson and white-Dominant Zeste Like elements into Sxl.